As means to form color images by transferring dyes to an image receiving element with the aid of heat, there are known a means to use a diffusion transfer heat-developable light-sensitive material and a means to use a thermal transfer material. Since these methods comprise transfer of dyes by heat energy, higher sensitivities (shortening of the transferring time) can be obtained by use of dyes which are highly diffusible on heating.
However, such highly diffusible dyes have disadvantages that transferred images are lowered in density during preservation due to rediffusion of the dyes, and that blurs are liable to occur owing to poor fixing capabilities of the dyes.
As preventive measures against such troubles, there have so far been proposed various methods for enhancing the fixation such as a method of adding mordants for the dyes to the image receiving element and a method of reacting the image receiving element with the dyes. As one of such proposals, Japanese Pat. O.P.I. Pub. Nos. 78893/1984 and 2398/1985 disclose a method for forming a chelated dye image by heating a thermal transfer material containing a heat-diffusible dye capable of forming a chelated dye to transfer the dye to an image receiving element and, thereby, allowing the dye to react with a metal ion providing compound contained in the image receiving element. Further, Japanese Pat. O.P.I. Pub. No. 197088/1991 discloses a metal ion providing compound improved in solubility, but the reactivity of forming a chelated dye is not satisfactory.
These methods are effective in preventing the lowering of dye image density and in improving the dye fixation, but have a disadvantage that a metal ion providing material can hardly be dispersed stably in an image receiving element. Particularly, when a metal ion providing compound is added to a hydrophilic binder, a preferred binder, to receive a thermal diffusible dye, the compound tends to deposit or aggregate and, thereby, causes uneven image densities which deteriorate the image quality. Further, since the metal ion providing compound itself is colored, the white background of an image gets colored, impairing the image quality. Moreover, when the chelation between metal ions and dyes is insufficient, the color tone of the dye is apt to be changed by the degree of chelation and, thereby, undesirable results are brought about in color reproduction. Accordingly, a high temperature or prolonged heating of transferred images becomes necessary to complete the chelation. And this involves another problem of making image forming apparatus complicated and expensive.